DC-side Current Research Articles

High-precision bus voltage control based on NLESO and TSMC predicted load current and grid voltage differential feedforward

Power battery test equipment is widely used to produce and test new energy vehicles and storage containers. To address the problem of DC bus voltage fluctuations that affect the accuracy of output voltage and current, a high-accuracy bus voltage control strategy is based on nonlinear extended state observation (NLESO) and terminal silding mode controller (TSMC) load current estimation. Simulation and experimental results show that the control strategy has a good suppression effect on the DC bus voltage fluctuation and can effectively improve the accuracy of the DC side output voltage and current. Grid voltage differential feedforward is adopted, which can achieve fast suppression of DC bus voltage fluctuations in power battery test equipment and meet the high precision output requirements of the system DC bus.

A New Carrier Phase-Shift Modulation Based on Switching the Displacement Angle

In this paper, a new carrier phase-shift (CPS) modulation method based on switching the displacement angle (SDA) is proposed to compromise the harmonic content of the output voltage and the circulating current. It can be used in medium- and low-voltage applications where the AC-side voltage and DC-side current of the modular multilevel converter (MMC) are required to have low harmonic content simultaneously. In this proposed SDA-based CPS modulation, the carrier displacement angle of the MMC with N submodules in each arm is periodically switched between the values of 0 and π/N degrees, so that the harmonic content of the output voltage and the harmonic content of the circulating current will not be in extreme conditions, which occurs when the displacement angle is set to 0 or π/N degrees. The effectiveness of this method has been verified by simulation and experimental results.

A High Power Density Multilevel Bipolar Active Single-Phase Buffer With Full Capacitor Energy Utilization and Controlled Power Harmonics

Active power decoupling in single-phase conversion systems reduces the required capacitance for twice-line frequency power ripple buffering by operating the energy buffering capacitors with high energy utilization ratios. The bipolar active buffer is the buffer topology that can achieve 100% energy utilization ratio. Yet, the conventional two-level full-bridge designs suffer from high switching and conduction loss and large filter inductor size due to the direct connection of the buffer across the high voltage dc-bus. Moreover, the high voltage swings across the buffer capacitor impose challenges for the control to regulate harmonics in the system. This article improves the power density and the efficiency of the bipolar active buffer through a buffer architecture incorporating the flying capacitor multilevel topology. Moreover, control techniques to regulate harmonics in the buffer are proposed to maintain low ripple in the dc-side current. The use of two common types of capacitors, the metal film capacitor and the X6S multilayer ceramic capacitor (MLCC) as buffer capacitors, is addressed with different harmonic control schemes. With MLCCs, we have also achieved a total passive component volume that is almost three times smaller than state-of-the-art solutions. The proposed approach is validated using a compact hardware for a 2-kW, 400-V dc-bus, 60-Hz single-phase inverter application.

Stator Winding Fault Diagnosis Method Based on Dc Side Current of Inverter

This paper analyzes the influence of the fundamental negative sequence component of the grid voltage on the output voltage of the variable frequency power supply, and points out that the fundamental negative sequence component of the grid voltage will not generate the fundamental negative sequence current component in the stator current of the induction motor, so when the stator winding fault is diagnosed, the influence of the negative sequence voltage of the grid does not need to be compensated. Based on the characteristic that the fundamental negative sequence current component in the stator current is converted into two times of the inverter output frequency component in the DC side current of the inverter, a stator fault diagnosis method based on the DC side current of the inverter is proposed. The method of connecting the short-circuit resistance between the two phases of the motor is used to simulate the nonmetallic short-circuit fault of the stator coil. The experimental results show that this method does not need to compensate the negative sequence voltage, and only needs to collect one current signal, and the algorithm is relatively simple, which is a simple and effective method.

Precise Calculation Method of Vector Dwell Times for Single-Stage Isolated Three-Phase Buck-Type Rectifier to Reduce Grid Current Distortions

Since the dc side current of the single-stage isolated three-phase buck-type rectifier (1S-ITBR) is discontinuous, the conventional current space vector pulsewidth modulation presents a poor grid current characteristics. In this article, the expressions of the instantaneous grid current of 1S-ITBR and the amplitudes of various-order grid current harmonics are originally derived. These results reveal the essential reason for causing t low-order current harmonics. Then, the improved calculation method of dwell times of the effective current space vectors with the goal of eliminating the grid current distortion is proposed to improve the grid current quality. The correctness of the harmonics analysis and the feasibility and validity of the proposed improved method are experimentally verified.

Analysis of the performance characteristics and arm current control for modular multilevel converter with asymmetric arm parameters

In this paper, the performance characteristics analysis and an improved arm current control of modular multilevel converter (MMC) with asymmetric arm parameters are presented. The performance characteristics analysis of MMC is conducted based on the average switching function current model, which demonstrates that fundamental and double frequency ripples exist in the arm circulating current under asymmetric arm parameter conditions. Besides, there will be dc and double frequency components in the ac side current, while fundamental and double frequency ripples also appear in the dc side current. According to the analysis results, the arm current control based on an improved PIR (Proportional Integral-Resonant) controller via the FAC (feedback adjustment control) method is proposed to suppress the asymmetric components with significantly enhanced dynamic response. Theoretical analysis shows that by introducing the integration and feedback loops of the FAC method to the conventional PIR controller, the inertia and damping effects are emerged in the system, which can help to enhance the system dynamic response during transient and achieve better suppression performance of the asymmetric components. Simulation cases verify the correctness of the theoretical analysis and the validity of the proposed control.

Improving Small-Signal Stability of an MMC With CCSC by Control of the Internally Stored Energy

The dc-side dynamics of modular multilevel converters (MMCs) can be prone to poorly damped oscillations or stability problems when the second-harmonic components of the arm currents are mitigated by a circulating current suppression controller (CCSC). This paper demonstrates that the source of these oscillations is the uncontrolled interaction of the dc-side current and the internally stored energy of the MMC, as resulting from the CCSC. Stable operation and improved performance of the MMC control system can be ensured by introducing the closed-loop control of the energy and the dc-side current. The presented analysis relies on a detailed state-space model of the MMC, which is formulated to obtain constant variables in steady state. The resulting state-space equations can be linearized to achieve a linear time invariant model, allowing for eigenvalue analysis of the small-signal dynamics of the MMC. Participation factor analysis is utilized to identify the source of the poorly damped dc-side oscillations, and indicates the suitability of introducing control of the internal capacitor voltage or the corresponding stored energy. An MMC connected to a dc power source with an equivalent capacitance, and operated with dc voltage droop in the active power flow control, is used as an example for the presented analysis. The developed small-signal models and the improvement in small-signal dynamics achieved by introducing control of the internally stored energy are verified by time-domain simulations in comparison to an electro-magnetic transient (EMT) simulation model of an MMC with 400 submodules per arm.

Improvement in Harmonic Reduction of a Zigzag Autoconnected Transformer Based 12-Pulse Diode Bridge Rectifier by Current Injection at DC Side

In this paper, to improve the harmonic reduction ability in the input line current of a 12-pulse diode bridge rectifier using zigzag autoconnected transformer with a dc side current injection technique is presented. Conventional 12-pulse rectifier does not meet the IEEE-519 standard and therefore current injection technique at dc side is used to shape the converter current to minimize the harmonic distortion in the input line current. The proposed converter configuration uses a zigzag autoconnected transformer with inherent ability to block zero-sequence components, which eliminates the requirement of a zero-sequence blocking transformer, and thereby reduces the overall rating of the converter. The current injection technique at dc side enables to alter the dc side currents, which further shapes the input line currents to near sinewave with total harmonic distortion of less than 1% and power factor close to unity. The proposed converter configuration is simulated and analyzed in MATLAB/Simulink. The effects of variation in the magnitude of injected current on the input line current harmonics in terms of power quality indices are analyzed. A prototype of the proposed converter configuration is developed, and experimental results are presented to validate the theoretical design, analysis, and simulation results.

Impedance Modeling of Three-Phase Voltage Source Converters in DQ, Sequence, and Phasor Domains

This paper presents a modular approach for the impedance modeling of three-phase voltage source converters (VSC) by representing the VSC dynamics using three-by-three transfer matrix in the dq, sequence, and phasor domains. The transfer matrix form simplifies the modeling process by separately modeling the ac and dc side dynamics, and describing the VSC dynamics independent of the ac and dc side networks. It also explicitly captures coupling among the dominant frequency components of the ac and dc side voltages and currents in the off-diagonal elements. Modeling of the VSC ac and dc side impedances, including the effects of the network on the other side of the VSC, is presented using the transfer matrix models. Transfer matrix based impedance modeling in the three domains and several stability analysis case studies are presented for a VSC-based HVDC station in an offshore wind farm. It is shown that the coupling between the ac and dc networks, and between the positive and negative sequence components of the three-phase quantities, play an important role in the low-frequency stability of the VSC. Impedance models and stability analysis predictions are validated using the wind farm simulations.

An Input Current Feedback Method to Mitigate the DC-Side Low-Frequency Ripple Current in a Single-Phase Boost Inverter

A boost DC/AC converter is popular in AC line-integrated energy storage systems (ESSs) based on low-voltage DC sources such as battery, fuel cell, or supercapacitor. The direct DC/AC power conversion in the boost inverter introduces a second-order harmonic ripple current at the DC side of the boost converter, which leads to internal heating of the energy storage devices and degradation of their lifetime. In this paper, a novel current feedback method is proposed to mitigate the second-order harmonic current component at the DC side. The proposed method is able to significantly reduce the second-order harmonic component in the DC-side current of the boost inverter without increasing other harmonic components. Performance of the proposed method is validated on a single-phase grid-connected DC/AC boost inverter-based battery ESS experimental prototype.

Fuzzy-Logic Sliding-Mode Control Strategy for Extracting Maximum Wind Power

A fuzzy-logic sliding-mode control strategy is proposed to capture the maximum wind energy and reduce the generator-side current harmonics for a direct-driven wind power system. The control strategy mainly consists of a fuzzy logic controller for generating the optimum dc-side current and a double integral sliding-mode current controller capable of accurately tracking the optimum dc-side current. A resonant filter is also incorporated into the fuzzy logic controller to eliminate the generator-side current harmonics. Detailed design procedure, existence, and stability conditions of the proposed control strategy have been thoroughly investigated and presented. The capability and effectiveness of the proposed control strategy have been validated by comparative experimental results.

English

FACTS technologies allow for improved transmission system operation with minimal infrastructure investment, environmental impact and implementation time compared to the construction of new transmission lines. FACTS provide the needed corrections of transmission functions in order to efficiently utilize existing transmission systems and therefore minimize the gap between the stability and the thermal level. Interline Power Flow Controller(IPFC) is a Voltage Source Converter(VSC) based FACTS controller for controlling power in multi-line system located at a substation. The main aim of the IPFC is to verify the power transfer capability between two or more lines and to establish the proposed voltage regulation function. In this paper, a current source converter topology based IPFC is proposed. In this structure, the dc-side current is regulated to a value larger than the peak value of the maximum line current. The injected voltage is controlled according to the desired reactive power compensation and management active power flow for master line. The decoupled state feedback control for the injected voltage with a separated dc current control is applied to the proposed system. The proposed IPFC has been simulated using the Matlab program.

High‐frequency modelling of a three‐phase pulse width modulation inverter towards the dc bus considering line and controller harmonics

Closed-form analytical formulas are provided to calculate the dc bus harmonics of a three-phase sinusoidal pulse width modulation (SPWM) inverter. The harmonic analysis is performed by using a double Fourier series approach to determine the dc bus current frequency spectrum. For an arbitrary modulation index and load power factor, the full harmonic components of the inverter dc side current are calculated. Based on the developed analytical model, an equivalent circuit is proposed for the inverter harmonic analysis towards the dc bus. Moreover, the impacts of line harmonics and zero sequence injection in controller towards the dc bus are presented. The results show that the 5th and 7th ac line harmonics on the dc side current is appearance of the 6th harmonic in the dc side. The impact of zero sequence injection to the controller on the dc side is negligible. In addition to analytical formulation, different simulations and extensive measurements performed which the results verified the presented analytical framework.

A Protection Scheme of Ac Side Faults of Dc Distribution System

Dc distribution systems are multiterminal systems characterized by complexity and mixed operation of ac and dc. Ac side faults are more likely to happen and may cause more serious consequences compared with dc transmission systems, thus must be considered seriously. In this paper, ac side faults of the prototype dc distribution system under high impedance grounding scheme are analyzed in detail. Fault characteristics of dc side voltages and currents under ac side faults are drawn and sorted. A protection scheme of ac side faults is proposed based on dc voltage and current measurements. The effectiveness of the protection scheme proposed is demonstrated by simulation under PSCAD/EMTDC.

Impact of Line Current Harmonics on the DC Bus Quality of a Three-Phase PWM Inverter

Three-Phase PWM inverters are widely used in many applications. The dc bus of a three-phase PWM inverter is subject of harmonics due to the inherent PWM switching operation that may impose severe consequences to the system. The impact of 5th and 7th harmonic components on the dc bus ripple is investigated in this paper. The analytical formulation and simulation method have been used to quantify the impact of these harmonics on the dc bus quality. The results show that there is a low frequency component with an order of 6 (6th harmonic) on the inverter dc side current which is explained and discussed here.

New Overall Control Strategy for Small-Scale WECS in MPPT and Stall Regions With Mode Transfer Control

This paper presents a new overall control strategy for small-scale wind energy conversion systems (WECS). The purpose of the proposed strategy is to utilize a maximum power point tracking control (MPPT) to maximize the captured energy when the wind speeds are below the rated speed. For high wind speed region; two stall controllers are developed: The constant speed stall controller which will limit the rotational speed of the generator to its rated value, and the constant power stall controller, which will regulate the captured power to be within the system rating. For the MPPT control, a modified perturb and observe (P&O) algorithm is utilized, where the dc side current is used as a perturbation variable and the dc-link voltage slope information is used to enhance the tracking speed and stability of the algorithm. For the speed and power regulation operation during high wind speeds, the system is controlled in the stall region to limit the rotational speed and the power of the generator. A stabilizing control loop is proposed to compensate for the stall region instability. A new mode transfer control strategy is developed to effectively control the transition between different modes of operation while ensuring the system stability without using any preknowledge of the system parameters. A 1 kW hardware prototype is developed and tested to validate the proposed new control strategy.